Conclusions
Genetic approaches yield valuable insights into olfactory information processing in peripheral and central nervous systems. Experiments in worms and flies have identified neuropeptide signaling as a probable gain control mechanism, enhancing the range of the olfactory sensory neurons. Moreover, peptides could also integrate internal state information into the olfactory circuit. In mice, tracing
studies using modified viruses have revealed a circuit linking neurons in the periphery with those in higher
brain regions. The topographic connectivity between olfactory sensory neurons and olfactory bulb glomeruli is retained in projections to the amygdala and anterior olfactory nucleus, but lost in the projections to piriform cortex. Odors, presented as component odorants or as complex mixtures, activate small numbers of pyramidal cells distributed throughout the piriform cortex. Interestingly, complex odors are not represented by the sum of the individual components in piriform cortex, suggesting a neural correlate for olfaction as a unitary sense. Finally, the piriform might function to make the representations
of each odor unique by restricting activity to a small number of pyramidal neurons, allowing learning and
experience to play a key role in odor perception.